Gearbox with a three-point mounting that reduces gearbox distortion

ABSTRACT

A gearbox comprises a gearbox housing containing a gear train whose shafts are supported by the housing. The housing is fastened to a mounting plate at exactly three attachment points. This three point attachment reduces distortion of the housing that may cause misalignment of individual meshing gears in the gear train, leading to premature failure of those gears transmitting high torque.

BACKGROUND

Certain industrial processes require the use of speed reduction geartrains to reduce the customary speed of electrical motors to a few tensof rpm or less. The output shaft gear of such a gear train is physicallyquite large, perhaps several feet in diameter. Such a gear train and thehousing that typically holds it will be referred to as a gearbox. Aninput shaft provides high speed low torque power.

The housing provides support for the bearings on which the gears' shaftsturn, and also protects the gears from damage and holds a gear lubricantsuch as gear grease. A typical housing comprises a case and a cover. Inone preferred design the gear train includes a pair of bevel gears forallowing an output shaft whose axis is at right angles to the inputshaft.

The gear train has an output gear perhaps several feet in diameter thatthe output shaft carries. The torque that the output gear provides inthese applications is often extremely large. For example, such a geartrain may be used to slowly rotate a large industrial component holdingmaterials with combined total weight of many tons. The large outputtorque of such a gear train produces on the teeth of both the outputgear and the pinion gear driving the output gear, contact stresses thatare extremely large as well.

Most gears are manufactured with teeth having cycloidal profiles, whichprovide for rolling line contact between meshing gears. Rolling contactavoids sliding between meshing teeth surfaces so little frictional wearoccurs. Line contact (as opposed to point contact) reduces the Hertzianstress (force per unit contact area) on these teeth and furtherincreases their life.

High precision machining now available for the teeth on such gearsprovides the opportunity for reliably creating during operation,continuous line contact between the individual teeth of the output gearand the meshing pinion gear. High precision bearings for the gear shaftsfurther enhance the opportunity for continuous precision meshing ofoutput gear teeth.

Unfortunately, output gear teeth continue to fail, often showing damageassociated with excessive Hertzian stress. Such failure is costly, inthat these gearboxes are very expensive. Further, it requires the entireinstallation to cease operation during gearbox or gear replacement.Since these gearboxes are often specially designed for a particularinstallation, replacement may take a long time. Alternatively, aninstallation may keep a spare gearbox available, reducing downtime butadding overhead costs to the operation of the installation.

Accordingly, the current state of affairs for installations using theselarge, high-torque gearboxes is unsatisfactory.

BRIEF DESCRIPTION OF THE INVENTION

Investigation suggests that the current practice of bolting high outputtorque gearboxes to a rigid mounting plate at four different points maydistort the gearbox case. Because of the extremely high output torquesinvolved, very strong attachment bolts are used to fasten the case tomounting points on the mounting plate.

The geometry of the mounting points on the mounting plate is typicallydifficult to control. When these mounting plate mounting points do notaccurately align with the mounting points on the gearbox and nocorrective shimming occurs, bolting the gearbox case to the mountingplate at four points can potentially distort the entire case. Thisdistortion of the case is small because the case is quite stiff comparedto the mounting plate, but still causes slight misalignment of themeshing gears within the case.

Such gearbox case distortion has to potential to alter the interfacebetween the high-torque output shaft gear and the gear driving theoutput shaft gear. This altered interface may lead to point or evenintermittent contact rather than line contact between the gear faces,causing premature damage to the gear faces, and eventual failure of thegears.

This invention limits gearbox distortion arising from the commonpractice of using four points for mounting the gearbox on the mountingplate. The gearbox is fastened to a mounting plate in a special way.

The invention comprises a gearbox of the type having a gearbox housingcontaining a gear train. The gear train has at least two shafts mountedfor rotation therein and supported by the gearbox housing. Each shaft ofthe gear train carries at least one gear in mesh with a gear carried onanother of the shafts. The improvement comprises on the exterior of thegearbox housing, exactly three active attachment points. Each attachmentpoint has at least one bolt engaging the attachment point for attachingthe housing to the mounting plate only at the attachment points.

Each attachment point preferably comprises a flange integral with thehousing and having at least one aperture such as a hole or a slotthrough which one of the bolts passes and is screwed into the mountingplate.

The invention is most valuable for a gearbox having a reduction geartrain having an output shaft turning much more slowly than an inputshaft. Such a reduction gear train has an intermediate shaft carrying anintermediate gear and an output shaft having an output gear rigidlyattached thereto with a preset number of teeth. The intermediate gear isin mesh with the output gear and drives the output shaft through theoutput gear. The intermediate gear has a fraction of the number ofoutput gear teeth.

A preferred gearbox has two flanges located closer to the input shaftthan to the output shaft.

BRIEF DESCRIPTION OF THE DRAWING

The FIGURE shows the invention with three mounting points for thegearbox housing.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The FIGURE shows a gearbox 10 having a housing 11 comprising a case 15and cover 13. Case 15 is to be attached to a mounting plate 21 usingbolts 51 that fit through holes in mounting points 18, 19, and 20 intoholes 46, 47, and 48 of mounting plate 21. Mounting points 18, 19, and20 preferably are all carried on a single flat surface of case 15, andpreferably on flanges forming a part of the bottom surface of case 15 asshown. Preferably, mounting surface 18, 19, and 20 project slightly pastthe surface carrying them to avoid any interference or contact betweencase 15 and mounting plate 21, other than at the individual mountingpoints 18, 19, and 20.

Gearbox 10 in one version contains a gear train having an input shaft 23and an output shaft 26. The gear train multiplies at output shaft 26,the torque applied to input shaft 23 by an overall ratio of 100 or more.

An output gear 37 is fixed to output shaft 26 and meshes with and isdriven by an intermediate gear 36. Gears 36 and 37 are shown in phantomoutline within gearbox 10; the intermediate gears comprising the geartrain other than gear are not shown. Intermediate gear 36 typically hasa fraction of the number of teeth that output gear 37 has. Torque istransmitted to output gear 37 from input shaft 23 through the one ormore intermediate gears in addition to gear 36. The gear train mayinclude bevel gears that orient the axis of the output shaft 26 to beorthogonal to the axis of input shaft 23, which is the configuration ofgearbox 10.

Bearing 43 mounts input shaft 23 and is supported by at least one wallof case 15. A shaft 33 attached between case 15 and cover 13 carriesintermediate gear 36.

Gear 36 through the gear train, applies relatively high torque to shaft26 through output gear 37, hence forces between the meshing tooth facesof both gears 36 and 37 are relatively high. Accordingly, anymisalignment between the meshing teeth of gears 36 and 37 has thepotential to create points of very high pressure on these teeth.

An internal bracket 30 (typically integral with or forming a part ofcover 13) carries the upper end of shaft 33, and also carries an upperbearing 32 for output shaft 26. A portion of internal bracket 30 isshown in phantom in the FIGURE. The floor of case 15 carries an innertriangular mounting plate or bracket 29 that in turn carries a secondoutput shaft bearing 40 also having low axial and radial runout.

As the FIGURE shows, thrust plate 29 is in the shape of a symmetrictrapezoid having long and short parallel sides and non-parallel sides ofequal length. The short parallel side of plate 29 is adjacent to andattached to mounting point 20. The long parallel side of plate 29extends between mounting points 18 and 19, and is attached to them.Bearing 40 lies on a line that may be drawn to bisect the long and shortparallel ends, i.e., is centrally mounted on plate 29. Thus, plate 29 issymmetrically arranged with the mount points 18, 19, and 20.

Both mounting bracket 29 and bearing 40 are spaced from the floor orbottom of case 15 to avoid damage from contaminants that settle to thefloor of case 15. Bracket 29 is solidly mounted to the floor of case 15,or is integral therewith, and has sufficient stiffness to distributehigh thrust and torque loads across a substantial portion of the case15. Brackets 29 and 30 are strongly attached to case 15 and cover 13respectively. Cover 13 is strongly attached to case 15. Because of this,any distortion of case 15 can also distort cover 13 and brackets 29 and30.

In one version, brackets 29 and 30 are designed to compensate for shaft26 deflection between bearings 32 and 40 by having different in-planestiffnesses. Thus, if gear 37 is positioned closer to bearing 32 than tobearing 40, then bracket 30 is proportionately stiffer than bracket 29so that the axis of shaft 26 when under maximum load is almost exactlyparallel with the shaft 26 axis when shaft 26 is unloaded.

To avoid premature failure of gear 37 and pinion gear 36, shafts 33 and26 should maintain their geometry respecting each other regardless ofthe torque that shaft 26 carries. The design of shaft 26 and bearings 32and 40 allows them to contribute only a very small amount of gear 37deflection arising from high torque transmission. This requires thatshafts 33 and 26 and housing 11 have substantial stiffness.

Case 15 has a mounting feature that substantially reduces distortion ofhousing 11 caused during attachment of housing 11 to a large, stiff basesuch as mounting plate 21. In the embodiment of the FIGURE, case 15 hastwo spaced apart mounting points 18 and 19 near the input shaft bearing43 and relatively distant from the axis of output shaft 26. Mountingpoints 18 and 19 may be integral with case 15 as shown, with holesthrough which mounting bolts 51 pass. A third mounting point 20, alsointegral with case 15, is spaced from mounting points 18 and 19 and maybe closer to the axis of output shaft 26. Bolts 51 fasten case 15 atthese three mounting points 18-20 by screwing into threaded mountingholes 46, 47, and 48 on mounting plate 21. Clamps that lock onto flangesor projections of case 15 are also possible fastener elements.

Using only the three mounting points 18, 19, and 20 appears tosubstantially reduce the distortion of housing 11 typically induced byimperfectly shimmed four-point mounting. Three point mounting, even ifthe individual mounting points are misaligned, distort the gear case 11only slightly at worst. On the other hand, a fourth mounting point mustbe accurately shimmed with the other three mounting points to avoid suchdistortion. For such shimming to be effective, the installer mustprecisely measure the gear case 11 and the mounting plate's matinggeometries. Since these gearboxes 10 are typically very heavy, mountingthem properly to avoid distortion is difficult and time-consuming.

The situation is similar to the performance of a three-legged stool onan uneven surface as opposed to a four-legged stool. A three-leggedstool will always sit solidly on an uneven surface. Everyone is familiarwith situations where a four-legged stool or table in a restaurant rockson an uneven floor, until matchbooks or folded napkins (i.e. shims) areplaced under one leg.

Three-point mounting of case 15 likely solves much of the case 15distortion problem. A further possible source for case 11 distortion mayarise from the high reactive torque that shaft 26 applies to case 11.That is, shaft 26 creates a couple tending to twist case 15.

The following may address this issue, if present. While the FIGURE showstwo mounting points 18 and 19 relatively distant from output shaft 23,alternatively two of the mounting points 18-20 may be placed closer tooutput shaft 26 and one closer to input shaft 23. This configuration maymake the overall installation more robust than that shown in the FIGURE.Placing the resistance to reactive torque applied by shaft 26 to case 15by a single one of the mounting points 18-20 as far from shaft 26 as thedimensions of case 15 allow, may also further reduce distortion of case15 arising from the high shaft 26 torque itself.

The invention claimed is:
 1. A gearbox assembly comprising: a gear traincharacterized by an input shaft and an output shaft, gears of said geartrain multiplying, at said output shaft, torque applied to said inputshaft by at least a factor of one-hundred, said output shaft including abearing; and a case within which said gear train resides, said casehaving a top surface, a bottom surface carrying a mounting plate, and atleast one wall connecting the top surface to the bottom surface; whereinsaid mounting plate carries said bearing of said output shaft, saidmounting plate including only three mounting portions, each mountingportion having a plurality of mounting holes, the holes in a first oneof the mounting portions being positioned along a first line, the holesin a second one of the mounting portions being positioned along a secondline, and the first and second lines being substantially perpendicularto each other.
 2. The assembly of claim 1, wherein the input shaftprojects out of the case through the at least one wall at a locationbetween said second and third mounting portions.
 3. The assembly ofclaim 1, wherein the top surface carries a second bearing for the outputshaft.
 4. The assembly of claim 1, wherein the output shaft projects outof the case through the top surface.
 5. The assembly of claim 1, whereinthe mounting plate is integrally formed with the bottom surface.
 6. Theassembly of claim 1, wherein the three mounting portions are formed byflanges projecting from the bottom surface.
 7. The assembly of claim 1,wherein the bottom surface of the case is flat.
 8. The assembly of claim1, wherein the top surface of the case is formed by a cover.